1. The Field of the Invention
The present invention generally relates to optoelectric systems and devices. More specifically, embodiments of the present invention relate to a transistor header assembly for use in an optical transceiver module, wherein the header assembly includes various features directed to monitoring and adjusting a laser device of the transceiver for optimum performance.
2. The Related Technology
Fiber-optic components and opto-electronics have become important components in modem networking circuits. The use of fiber-optic circuits allows for efficient, accurate and quick transmission of data between various components in a network system.
As with the design of most any system, there are engineering tradeoffs that often have to be made when implementing fiber optic systems. For example, the size and modularity of components must often be balanced against the need for additional space to accommodate heat dissipation and circuit monitoring components. While it is desirable to minimize a component's size, some configurations have previously limited this minimization due to their inherent characteristics. For example, previously many lasers used in fiber-optic systems that have the characteristics needed for long-distance transmission and/or dense wavelength division multiplexing (DWDM) generated amounts of heat that could not be dissipated by some smaller package sizes. Further, smaller package sizes have a limited amount of space available for mounting and connecting additional components such as thermistors, monitor photodiodes, thermoelectric coolers, or impedance matching circuits.
Regarding smaller package sizes, it is desirable in fiber optic systems to use modular components so that a system can be created in a compact area and with as little expensive customization as possible. For example, many fiber optic systems are able to use modular transceiver modules. The modular transceiver modules include an input receiver optical subassembly (ROSA) and an output transmitter optical subassembly (TOSA). The ROSA comprises a photodiode for detecting optical signals and sensing circuitry for converting the optical signals to digital signals compatible with other network components. The TOSA comprises a laser for transmitting optical signals and control circuitry for modulating the laser according to an input digital data signal. The TOSA has an optical lens for focusing the light signals from the laser of the TOSA to an optical fiber. Additionally, the transceiver module includes pluggable receptacles for optically connecting the TOSA and the ROSA with other components within a fiber optic network.
The transceiver module often includes an electronic connector for connection to electrical components of the computer or communication device with which the transceiver module operates (a “host system”). The design of the transceiver, as well as other components within the fiber optic system, is standards-based, such that components can be connected without significant customization.
One particular pluggable standard that is currently being developed is the 10-Gigabit Small Form-factor Pluggable (XFP) standard. This standard defines various characteristics such as size, power consumption, connector configuration, etc. With regards to power consumption, the XFP standard references three power consumption levels of 1.5 W, 2.5 W and 3.5 W. When designing devices to operate within the XFP standard, attention must be given to what components are selected and how they are configured so as to not exceed the rated power consumption. These devices are constrained by principles of semiconductor physics to work preferentially in a certain temperature range. The module power dissipation and the package size and materials uniquely determine the module operating temperature for given ambient conditions, such as ambient temperature, airflow, etc. The resulting module operating temperature determines the types of optical and electronic components that can be successfully operated within the package. One such package is known as a transistor-outline header, otherwise known as a TO can or TO.
Transistor-outline headers are widely used in the field of opto-electronics, and may be employed in a variety of applications. As an example, transistor headers are sometimes used to protect sensitive electrical devices, and to electrically connect such devices to components such as printed circuit boards (“PCB”).
With respect to their construction, transistor headers often consist of a cylindrical metallic base with a number of conductive leads extending completely through, and generally perpendicular to, the base. With regard to the metallic base, the size of the base is often sized to fit within a specific TO standard size and lead configuration, examples of which include a TO-5 or TO-46. The leads are hermetically sealed in the base to provide mechanical and environmental protection for the components contained in the TO package, and to electrically isolate the conductive leads from the metallic material of the base. Typically, one of the conductive leads is a ground lead that may be electrically connected directly to the base.
Various types of devices are mounted on one side of the base of the header and connected to the leads. Generally, a cap is used to enclose the side of the base where such devices are mounted, so as to form a chamber that helps prevent contamination or damage to those device(s). The specific characteristics of the cap and header generally relate to the application and the particular device being mounted on the base of the header. By way of example, in applications where an optical device is required to be mounted on the header, the cap is at least partially transparent so to allow an optical signal generated by the optical device to be transmitted from the TO package. These optical TO packages are also known as window cans.
Although transistor headers have proven useful, typical configurations nevertheless pose a variety of unresolved problems. Some of such problems relate specifically to the physical configuration and disposition of the conductive leads in the header base. As an example, various factors combine to compromise the ability to precisely control the electrical impedance of the glass/metal feedthrough, that is, the physical bond between the conductive lead and the header base material. One such factor is that there are a relatively limited number of available choices with respect to the diameter of the conductive leads that are to be employed. Further, the range of dielectric values of the sealing glass typically employed in these configurations is relatively small. And, with respect to the disposition of the conductive leads, it has proven relatively difficult in some instances to control the position of the lead with respect to the through hole in the header base.
Yet other problems in the field concern those complex electrical and electronic devices that require many isolated electrical connections to function properly. Typically, attributes such as the size and shape of such devices and their subcomponents are sharply constrained by various form factors, other dimensional requirements, and space limitations within the device. Consistent with such form factors, dimensional requirements, and space limitations, the diameter of a typical header is relatively small and, correspondingly, the number of leads that can be disposed in the base of the header, sometimes referred to as the input/output (“I/O”) density, is relatively small as well.
Thus, while the diameter of the header base, and thus the I/O density, may be increased to the extent necessary to ensure conformance with the electrical connection requirements of the associated device, the increase in base diameter is sharply limited, if not foreclosed completely, by the form factors, dimensional requirements, and space limitations associated with the device wherein the transistor header is to be employed.
In connection with the foregoing, another limiting aspect of many transistor headers relates to the relatively limited physical space available on the header. In particular, the relatively small amount of space on the base of the header imposes a practical limit on the number of components that may be mounted thereon. To overcome that limit, some or all of any additional components desired to be used must instead be mounted on the printed circuit board, some distance away from the laser or other device contained within the transistor header. Such arrangements are not without their shortcomings however, as the performance of active devices in the transistor header, such as lasers and integrated circuits, depends to some extent on the physical proximity of related electrical and electronic components. By minimizing the distance between the lasers and integrated circuits to the additional components required for impedance matching, the inherent transmission line between such components is minimized. As such, placing the components in close physical proximity reduces reflective transmission line losses.
In addition to the above, further challenges are faced when certain functions are desired to be added to the transistor header. In particular, it is beneficial to monitor and/or modify the character of the optical signal that is produced and emitted by the laser device of the TOSA. For instance, the optical signal that is output by the laser device can change in power and/or wavelength over time as the laser heats up within the TOSA during operation of the optical transceiver module. If these laser parameters are actively monitored during transceiver operation, adjustment can be made to the laser device to ensure its proper operation.
Monitoring of laser device parameters such as power and output wavelength is typically accomplished using one or more monitor photodiodes (“MPD”). In the case of laser power monitoring, a single MPD is often used, while wavelength monitoring typically requires two MPDS working in concert. In either case, it is desirable to locate the one or more MPDs within the transistor header assembly in close proximity to the laser device so that laser light produced by the laser device can be received by the MPDs. However, as has already been explained, the location of optoelectronic devices within the transistor header assembly requires additional electrical interconnects to be passed through the header assembly, which may not be possible in known header assembly designs that must comply with prescribed dimensional form factors. Thus, the use of one or more MPDs within a transistor header assembly may be restricted or foreclosed completely.
In light of the above discussion, a need exists for a transistor header design for use within an optical transceiver module that overcomes the above challenges. In particular, a transistor header is needed that enables a relatively greater number of interconnects to be established between the header interior and devices located outside of the header. Such a solution should also enable the utilization of monitoring devices, such as one or more monitor photodiodes, within the transistor header assembly to optimize the operation of optoelectronic components, such as a laser device, located therein.